1. Field of the Invention
The present invention relates generally to a hot wire type air flow meter for use in an automotive internal combustion engine and more specifically to such a device which includes an efficient self-cleaning arrangement which is capable of self-timing its operation.
2. Description of the Prior Art
FIGS. 1 and 2 show a prior art arrangement disclosed in Japanese Patent Application First Provisional Publication No. 59-20813 published on Feb. 2, 1984. This arrangement has attempted to endow a hot wire type air flow meter 1 with self-cleaning characteristics by arranging for a current to be passed through the sensing element (hot wire) 2 of the air flow meter each time the engine 3 is stopped. The object of this arrangement is to vaporize fuel and the like which tends to accumulate thereon. Viz., when the engine 3 is stopped, as the engine is still hot, the fuel which inevitably wets the walls of the induction system downstream of the point at which fuel is supplied thereinto by a fuel injector 4, evaporates and tends to pass back around an engine throttle valve 5 and subsequently condenses at locations upstream thereof. This, in combination with the fine dust and the like which tends to enter the induction system despite the provision of an air cleaner 6, tends to cause thermally insulating deposits to accumulate on the sensor element (hot wire) and thus reduce the accuracy of the device.
In order to promote effective cleaning the circuitry 7 which is associated with the hot wire sensor element includes a timer arrangement which delays the supply of electrical energy to the wire for a predetermined time t (see FIG. 2) after the ignition switch is opened.
However, depending on the mode of engine operation immediately prior the driver turning the ignition key to a position which will stop the operation of the engine, the time required for the air in the induction system in the vicinity of the flow sensor to assume a zero velocity and for the fuel vapor to disperse and/or condense, can be as long as 4 seconds.
During cleaning operation, if the wire is excessively cooled by movement of air and/or the presence of fuel vapor it tends to be insufficiently cleaned. On the other hand, if the movement of the air stops very quickly then the possibility of it being overheated in a manner which reduces the longevity thereof exists.
Thus, this arrangement has suffered from the drawback that it is very difficult to suitably select the fixed delay in view of the various variables involved in a manner which promotes the required cleaning efficiency and which avoids the risk of unduly reducing the life of the sensor.